1. Field of the Invention
This invention relates to the evaluation of residual stresses in metal components; more particularly, to a method for determining biaxial stresses in steel components.
2. Description of the Prior Art
Stress and structural defects in ferromagnetic materials such as steel, as well as certain other properties of the metal piece, may be identified by creating a time-varying magnetic field within the metal piece and analyzing the magnetic noise created in the metal piece by the magnetic field. This detection technique employs a phenomenon known as the "Barkhausen effect" which has become well known to workers in the art as a series of abrupt changes or jumps in the magnetization of a substance when the magnetizing field created in the metal piece is gradually altered. An excellent background discussion of the Barkhausen effect appears in Leep et al. U.S. Pat. No. 3,427,872.
The abrupt jumps that occur in the magnetization as the intensity of the field is changed can be detected as electrical noise by a sensing coil disposed proximate or in contact with the metal piece. Several sensor configurations are described in Tiitto U.S. Pat. No. 4,634,976. The noise carried by the electrical leads from the sensing coil, commonly referred to as "Barkhausen noise", can be fed through a suitable processing network and, if desired, to an audio speaker. The level of the Barkhausen noise that is generated at a location within a metal piece depends in part on the sense, magnitude and direction of the stress or strain at that location and the microstructure of the metal. Accordingly, workers in the art have attempted to employ the Barkhausen effect and Barkhausen noise to identify stresses or strains and defects in and some microstructural characteristics of a metal piece.
Most systems that employ the Barkhausen effect to identify stresses or strains and defects in a metal piece include an energizing coil assembly and a sensing coil assembly. The energizing coil is disposed proximate the location of the metal piece under examination and is energized with a periodically time-varying signal to induce in the metal piece a periodically time-varying magnetic field. The resultant Barkhausen noise generated in the metal piece is sensed by the sensing coil and fed to circuitry which can process the noise in a variety of ways. Ultimately, the processed Barkhausen noise is fed to a device for display. The Barkhausen noise is referred to herein in terms of an MP (magnetoelastic parameter) value, which is proportional to the level of Barkhausen noise measured within a specified frequency range.
In conventional practice of testing stresses in a metal piece, Barkhausen noise levels (MP values) are put to practical use by first preparing uniaxial calibration curves from test pieces of each grade of steel to be examined; making actual measurements of MP values in the steel component to be tested; and comparing the MP values derived from the actual measurements against the appropriate uniaxial calibration curve to determine the stresses or strains present at the test location on the steel component. A uniaxial calibration curve is obtained by stressing a test piece in compression and tension in one (axial) direction only and ignoring the stresses in other directions or assuming that they are very small. The MP value measured under each condition of compression or tension constitutes a representation of the stress/strain present along the axis of loading. Such measurements are valid as long as they are performed within the elastic region of the test piece.
Workers in the art have reported wide variations between stress/strain measurements made by the Barkhausen noise calibration curve method just described and other stress/strain measurement methods. In most cases, it has been assumed that such variations can be related to differences in the microstructural variables of steel, such as composition, texture, grain size, deformation and metallurgical structure. However, virtually all practical steel components (i.e., welded, fabricated components) have a complex biaxial residual stress condition created by the steel manufacturing process, and enhanced and modified by subsequent fabrication, welding and machining processes; these are the important residual stresses that manufacturers and users of steel components seek to determine. As will be apparent from the description that follows, application of uniaxial calibration to test biaxial stress/strain condition may lead to errors that may be larger than those resulting from typical microstructural variation within a specified steel grade.
The concept of biaxial, or plane, stress/strain condition is well understood. According to the linear elasticity theory and Hooke's law, every plane strain condition can be expressed in terms of two principal normal strains (.epsilon..sub.1, .epsilon..sub.2) which are perpendicular to one another, and one principal out-of-plane normal strain (.epsilon..sub.3) perpendicular to the surface: ##EQU1## where x and y are randomly selected coordinates in the sample plane, .epsilon..sub.x, .epsilon..sub.y are normal strains measured in x and y directions, .gamma.xy is the shear strain in the xy sample plane; and .nu.is the Poisson ratio.
Two principal normal stresses (.sigma..sub.1, .sigma..sub.2) of biaxial plane stress condition may be easily found by the expressions: ##EQU2## where E is modulus of elasticity. It may be noted that, by definition, a free surface cannot support normal stresses perpendicular to the surface plane, i.e., .sigma..sub.3 =0; normal strain .epsilon..sub.3 may still exist, however, due to Poisson's effect.
For engineering applications, strains .epsilon..sub.1, .epsilon..sub.2 and stresses .sigma..sub.1, .sigma..sub.2 are commonly evaluated using a blind hole drilling method or X-ray diffraction. Both of these techniques are often considered time consuming and less practical.
The conventional application of uniaxial calibration, using Barkhausen noise as described above, to test biaxial plane stress/strain conditions may lead to serious errors, sometimes in the order of magnitude of 50-100%. If, for example, in employing the uniaxial calibration method, a certain MP value is measured on a structural component, that MP value is applied to a calibration curve, which assumes transverse strain (.epsilon..sub.2) to be zero, to yield a certain value of longitudinal strain (.epsilon..sub.1). Actually, however, the transverse strain in a structural component may have a large value, and the use of a calibration curve generated at the actual value of transverse strain would produce a longitudinal strain markedly different from the value derived from the use of the aforementioned calibration curve that assumes transverse strain to be zero. It is this possibility of both principal strain components being large that creates concern for manufacturers and users of structural components and that makes the uniaxial calibration method unreliable or even dangerous.
Accordingly, there exists a need to develop a Barkhausen noise method for better determining biaxial stress conditions in practical steel components. To date, there has been little recognition by workers in the art that biaxial residual stress distribution may actually be a major factor affecting Barkhausen noise testing. For example, in an article entitled "Barkhausen Biaxial Stress/Strain Measurement System" by K. Loomis, Proceedings of the 13th International Nondestructive Testing Conference (San Antonio, Texas 1981), data is provided from experiments in which there was little transverse stress in the principal directions under evaluation; from that data, the conclusion is advanced that no correction for biaxial stress is needed. In an article entitled "Detection of Fabrication Stresses by the Barkhausen Noise Method" by L. Karjalainen et al., The Effects of Fabrication Related Stresses on Product Manufacture and Performance (The Welding Institute, Abington Hall, Cambridge 1985), .sctn. 13, p. 1, the authors acknowledge the importance of biaxial residual stresses and present data on their influence on uniaxial calibration curves, yet suggest no procedure for evaluating the effects of the presence of biaxial residual stresses. In an article entitled "Magnetic Barkhausen Noise Analysis in Bi-Axial Stress Test" by Furuya et al., Journal of Japanese Society of Nondestructive Inspection, Vol. 36, No. 8 (1987), p. 530, the authors make an attempt to directly evaluate biaxial stresses with the Barkhausen noise method, but conclude that the noise can be related only to the difference in each principal stress.